diff --git a/test/test_mnist.py b/test/test_mnist.py
index 2eef2df1..5c0bd6b4 100644
--- a/test/test_mnist.py
+++ b/test/test_mnist.py
@@ -56,6 +56,7 @@ class TinyConvNet:
 def train(model, optim, steps, BS=128, gpu=False):
   losses, accuracies = [], []
   for i in (t := trange(steps, disable=os.getenv('CI') is not None)):
+    optim.zero_grad()
     samp = np.random.randint(0, X_train.shape[0], size=(BS))
 
     x = Tensor(X_train[samp].reshape((-1, 28*28)).astype(np.float32), gpu=gpu)
diff --git a/test/test_ops.py b/test/test_ops.py
index cf7b0f53..0ce6ac84 100644
--- a/test/test_ops.py
+++ b/test/test_ops.py
@@ -63,6 +63,8 @@ class TestOps(unittest.TestCase):
     helper_test_op([(45,65)], lambda x: torch.nn.LogSoftmax(dim=1)(x), Tensor.logsoftmax, atol=1e-7, grad_atol=1e-7, gpu=self.gpu)
   def test_tanh(self):
     helper_test_op([(45,65)], lambda x: x.tanh(), Tensor.tanh, atol=1e-6, grad_atol=1e-6)
+  def test_topo_sort(self):
+    helper_test_op([(45,65)], lambda x: (x+x)*x, lambda x: x.add(x).mul(x), atol=1e-6, grad_atol=1e-6)
 
   def test_broadcast_full(self):
     for torch_op, tinygrad_op in [(torch.add, Tensor.add), (torch.sub, Tensor.sub), (torch.mul, Tensor.mul),
diff --git a/tinygrad/gradcheck.py b/tinygrad/gradcheck.py
index 71aa9739..e56873c5 100644
--- a/tinygrad/gradcheck.py
+++ b/tinygrad/gradcheck.py
@@ -11,6 +11,8 @@ def jacobian(func, input):
   J = np.zeros((jo,ji), dtype=np.float32)
 
   for o in range(jo):
+    input.grad = None
+    output=func(input)
     # tinygrad doesn't support slicing, tiny-hack to select
     # the needed scalar an backpropagate only through it
     o_scalar = Tensor(mask_like(output.data, o, 1.)).mul(output).sum()
diff --git a/tinygrad/optim.py b/tinygrad/optim.py
index 99bd6534..a78f860d 100644
--- a/tinygrad/optim.py
+++ b/tinygrad/optim.py
@@ -7,6 +7,10 @@ class Optimizer:
   def __init__(self, params):
     self.params = params
 
+  def zero_grad(self):
+    for param in self.params:
+      param.grad = None  # PyTorch defaults to set to 0
+
 class SGD(Optimizer):
   def __init__(self, params, lr=0.001):
     super(SGD, self).__init__(params)
diff --git a/tinygrad/tensor.py b/tinygrad/tensor.py
index e77a5586..3cc741d4 100644
--- a/tinygrad/tensor.py
+++ b/tinygrad/tensor.py
@@ -109,7 +109,6 @@ class Tensor:
     return Tensor(np.eye(dim).astype(np.float32))
 
   def backward(self, allow_fill=True):
-    #print("running backward on", self)
     if self._ctx is None:
       return
 
@@ -119,21 +118,30 @@ class Tensor:
       assert self.data.shape == (1,)
       self.grad = Tensor(np.ones(self.data.shape, dtype=self.data.dtype), gpu=self.gpu)
 
-    assert(self.grad is not None)
+    visited, nodes = set(), []
+    def deepwalk(node):
+      visited.add(self)
+      if node._ctx:
+        for i in node._ctx.parents:
+          if i not in visited:
+            deepwalk(i)
+        nodes.append(node)
+    deepwalk(self)
 
-    with ProfileOp(self._ctx.__class__.__name__, [self.grad], backward=True):
-      grads = self._ctx.backward(self._ctx, self.grad.data)
-    if len(self._ctx.parents) == 1:
-      grads = [grads]
-    for t,g in zip(self._ctx.parents, grads):
-      if g is None:
-        continue
-      if g.shape != t.data.shape:
-        print("grad shape must match tensor shape in %r, %r != %r" %
-          (self._ctx, g.shape, t.data.shape))
-        assert(False)
-      t.grad = Tensor(g)
-      t.backward(False)
+    for t0 in reversed(nodes):
+      assert (t0.grad is not None)
+      with ProfileOp(t0._ctx.__class__.__name__, [t0.grad], backward=True):
+        grads = t0._ctx.backward(t0._ctx, t0.grad.data)
+      if len(t0._ctx.parents) == 1:
+        grads = [grads]
+      for t,g in zip(t0._ctx.parents, grads):
+        if g is None:
+          continue
+        if g.shape != t.data.shape:
+          print("grad shape must match tensor shape in %r, %r != %r" %
+            (self._ctx, g.shape, t.data.shape))
+          assert(False)
+        t.grad = Tensor(g) if t.grad is None else (t.grad + Tensor(g))
 
   # ***** tinygrad supports CPU and GPU *****